Reducing Tree Root and Sidewalk Conflicts: Analysis and

Reducing Tree Root and Sidewalk Conflicts:

Analysis and Strategies for the

City of Palo Alto

Submitted to: City of Palo Alto

Elise Willis, Urban Forestry Technician

October 5, 2016

Reducing Tree Root and Sidewalk Conflicts: Analysis and Strategies for the City of Palo Alto 2

Table of Contents

Acknowledgements ......................................................................................................................... 3

1.0 Abstract ..................................................................................................................................... 3

2.0 Introduction ............................................................................................................................... 4

3.0 Key Findings ............................................................................................................................. 5

3.1 City of Palo Alto Public Works Sidewalk Repair Program .................................................. 6

3.2 Tree-based, Infrastructure-based, and Rootzone-based Solutions ........................................ 7

3.2.1 Tree-based solutions ..................................................................................................... 12

3.2.2 Infrastructure-based solutions ....................................................................................... 14

3.2.3 Rootzone-based solutions ............................................................................................. 25

4.0 Recommendations ................................................................................................................... 30

4.1 City Processes, Planning, and Prioritization ....................................................................... 30

4.2 Decision Trees and Best Management Practices................................................................. 33

4.2.1 Decision Trees .............................................................................................................. 33

4.2.2 Best Management Practices (BMPs) ............................................................................ 36

4.3 Strategy Packages ................................................................................................................ 37

4.3.1 Capital Improvement Plans (CIP)................................................................................. 37

4.3.2 Development Review ................................................................................................... 39

4.3.3 Sidewalk Repair Program ............................................................................................. 42

5.0 Conclusion .............................................................................................................................. 43

5.1 Action Items ........................................................................................................................ 43

6.0 References ............................................................................................................................... 44

7.0 Appendices .............................................................................................................................. 46

7.1 Decision Trees and Sample Assessment Forms .................................................................. 46

7.2 Standard Details and Specifications .................................................................................... 49

7.3 Material and Vendor Resources .......................................................................................... 56


Acknowledgements

We thank the City of Palo Alto for funding this analysis, and the guidance provided by Walter

Passmore and Courtney Schumm during Milestone meetings. Holly Boyd, Bill Croft, and Jim

Amores provided valuable information on the Sidewalk Repair Program. Thanks also to staff

who helped to identify opportunities for incorporating new solutions into CIPs (Sarah Syed and

Kirsten Struve) and Development Review (Dave Dockter).

Valuable first-hand accounts of experience with many of the products and procedures were

provided by Gordon Mann (Mann Made Resources), Larry Costello (Oracle Oak LLC), Matthew

Fried (City of San Mateo), and Peg Staeheli (MIG/SvR).

Thanks also to Catherine Martineau (Canopy) for her support through this project.

1.0 Abstract

The conflicts between tree roots and sidewalks in Palo Alto are widespread, resulting in

recurring expenses for repair, injurious practices to offending trees, and premature tree removals.

Alternative solutions exist, but are not always utilized. This report presents potential solutions

and management recommendations to reduce conflicts between tree roots and sidewalks.

Evaluations of tree-based, infrastructure-based, and rootzone-based solutions are provided with

cost comparisons and approximate expected useful life. Decision trees are recommended to

enhance operations procedures in project planning and implementation. And finally, the

solutions are grouped into Strategy Packages in order to inform decision-making at the early

stages of Capital Improvement Plans (CIP), Development Review, and the Sidewalk Repair

Program.

This document is intended primarily for internal use by the City of Palo Alto staff - Public

Works, Planning and Community Environment, Utilities, Transportation, Parks and Recreation,

and Development Services offices. The Urban Forestry and Engineering divisions within Public

Works may use this as a resource for management and planning the Sidewalk Repair Program,

CIPs, and other activities requiring street tree and sidewalk maintenance.


2.0 Introduction

The urban environment is a web of natural and built features interacting daily. The sun beats

down on sidewalks, streets, and parking lots. Rain races off asphalt and saturated lawns into the

gutters. Buildings bake in the summer and freeze in the winter. Cars zip around spewing fumes

and exhaust. These are the daily workings of life in a city, and are all interrupted by an important

piece of infrastructure - trees.

Trees are combatants of the urban heat island effect, providing necessary shade to streetscapes.

Trees are interceptors of rainwater, providing stormwater retention for overloaded drainage

systems. Trees are absorbers of air and water pollutants, improving health and wellness in

neighborhoods. Trees are beautiful green canopies housing wildlife and improving the character

of communities. Trees are an integral piece of green infrastructure surrounded by grey, and their

benefits have grabbed the attention of leaders who are working to make them a priority in cities

around the world.

The City of Palo Alto Urban Forest Master Plan Survey responses spurred the need for this

study. When Palo Alto residents were asked what they do not like about the private and public

trees around them, 51% responded “damage caused by tree roots” - the highest percentage out of

all the options provided. Many Palo Alto city staff and residents are open to utilizing innovative

solutions that will improve sidewalk conditions while simultaneously supporting long-term

health and growth of trees.

The i-Tree streets analysis revealed that Palo Alto’s Benefit-Investment Ratio (BIR), which

compares the cost of tree planting and maintenance to the annual benefits it provides, is 3.22:1.

This high BIR means that for every $1 that the City spends on street trees, the City reaps $3.22 in

benefits (Davey Resource Group, 2010). “Unlike traditional infrastructure, such as pipelines,

buildings and roadways, urban forests appreciate in value over time, meaning a low-cost solution

now in the form of urban forest investments becomes a long-term benefit” (American Forests,

2014). This is significant, because with every tree prematurely removed the City is losing

benefits before return on investment can be fully achieved.

However, the services of trees do not outweigh the services of other city frameworks, because

they enhance the quality of life for people by working together. In 2011 Palo Alto’s City Council

adopted recommendations to reach a Pavement Condition Index (PCI) of 85 citywide as soon as

possible. The Public Works Engineering group is aiming for a PCI of 60 or above for every

street, and in order to achieve this goal they need to incorporate creative solutions that reduce

conflicts between trees and infrastructure. In addition, the Americans with Disabilities Act

(ADA) and the US Access Board Public Right-of-Way Accessibility Guidelines (PROWAG)

provide guidelines and regulations that ensure sidewalks in the public right-of-way and public


spaces are accessible, continuous, unobstructed for people. These activities are of high priority

for city staff and residents.

Palo Alto has over 29,000 street trees providing annual benefits of over $6.6 million ($103.73

per capita), and the Public Works Department’s (PWD) Urban Forestry group is responsible for

maintaining the trees in the public right-of-way. The annual investment required to care for these

resources is around $2.1 million, but the community receives a net benefit of over $4.5 million in

ecosystem services (Davey Resource Group, 2011).

The Public Works department also estimates that Palo Alto has over 250 miles of sidewalk, with

the city’s Sidewalk Repair Program costs exceeding $26 million over the last 29 years. The new

sidewalk repair and replacement budget for fiscal year 2017 alone is $1.5 million. Requirements

for ADA and PROWAG compliance mandate the PWD to uphold regulations for sidewalks in

the public right-of-way, and to maintain these sidewalks per the Palo Alto Municipal Code.

This report offers potential solutions and management recommendations to reduce conflicts

between tree roots and sidewalks. Many of these strategies and tools are being employed by

other cities throughout the US, and each is provided with brief descriptions, cost comparisons,

and the expected useful life. We also clarify processes and procedures currently used in Palo

Alto, and offer sample decision trees and strategy packages.

3.0 Key Findings

At the beginning of this analysis, a conference call was held with City of Palo Alto staff and

other urban forestry and arboriculture professionals from cities in California. To jump-start the

exploration of potential solutions, the group members took turns listing familiar strategies,

including alternative sidewalk base materials, alternative surface materials, and the importance

of providing proper rootable soil and space for trees. Some strategies were specifically named,

while others were deemed possible but needing more research.

Interviews were also conducted one on one in order to further understand Palo Alto’s past and

present processes and experiences with sidewalk and tree root management. Staff and Managers

for the Public Works Department explained some of the relevant city processes and requirements

for sidewalk maintenance and tree protection described below.

Literature review included influential publications and management documents from other cities.

The Seattle Department of Transportation (SDOT) completed their Trees and Sidewalks

Operations Plan in 2015, which presents the extremely relevant and up to date Solutions Toolkit

referenced throughout this report. Reducing Infrastructure Damage By Tree Roots: A

Compendium of Strategies by Larry Costello and Kristin Jones also elucidates many potential

solutions and provides samples of standard drawings used in various cities. These, among some


other articles, provided the basis for the strategies listed and should be used for more information

on specific solutions.

3.1 City of Palo Alto Public Works Sidewalk Repair Program

The City of Palo Alto Public Works Sidewalk Repair Program budget for Fiscal Year (FY) 2017

(July 1, 2016 - June 30, 2017) is up to $1.5 million per year with more emphasis on creating

handicap ramps and addressing the backlog in sidewalk repair and replacement work. CIP for

curb and gutter repair and replace is separate from the sidewalk repair, with a budget of $25,000

per year. Interestingly, it has generally been cheaper to do “piece by piece” sidewalk repair and

replacement than doing a large area all at once.

In 1986 Palo Alto created a management plan that divided the city into 23 districts where all of

the sidewalks were to be assessed and repaired over a period of 30 years. A Request for Proposal

(RFP) for FY2017 will propose to contract out the sidewalk work and no longer work “District

by District,” but likely work by highest priority across the city. A consultant will be selected to

help review the previous management plan and recommend how the city should proceed into the

future.

Jim Amores, City of Palo Alto Engineer, has overseen the Sidewalk Repair Program for the last

30 years, and had valuable insight and observations about the program. He has witnessed that the

most common result of sidewalk and tree root conflicts included lifting at the expansion joints

(concrete poured in lengths of 60 feet) and deep joints (located every 20 feet) in the sidewalk.

For the last few years, the process has simply been for the contractor to call Bill Croft (Urban

Forestry Arborist) to inspect if the sidewalk repair/replacement requires special care while

working around a tree. This case by case predominantly reactionary inspection process usually

requires staff to visit sites multiple times per day on average. Instances of special care typically

include determining whether to prune roots 4 inches or larger, increasing the sidewalk radius

around the trunk (reducing the sidewalk width), or routing the entire new sidewalk section to be

farther from the tree trunk (moving part of the sidewalk onto the adjacent property). Bill will also

perform a pull test to determine if too many roots have been cut, and if so, will schedule tree

removal and replacement by his crew.

Probably the most common “tree-centric” remedial method for sidewalk work is to saw the

sidewalk to curve away from the trunk, which reduces the sidewalk width and increases the area

in the planter. City staff and contracting companies doing the work have a Specifications (Spec)

Book on site. The Spec Book specifies that PW Urban Forestry (Bill Croft) should be called

prior to cutting any roots 4” or larger.


How does Engineering determine the fix needed (i.e. repair or replace the sidewalk)? Jim walks

the sidewalk and spray paints areas that need repair, replace, or remove and no replace. If the

sidewalk is raised by ¾ inch or more it is considered a tripping hazard and needs replacement. If

the sidewalk is raised by ¼ inch to ¾ inch it is technically not a tripping hazard, and therefore

must at minimum be ground down or beveled with asphalt. ADA requires that the minimum

allowable sidewalk width is 4 feet, which limits available solutions in the field.

Oscar Godinez is the Manager of Maintenance Operations for the PWD Operations Storm

Drainage, and he evaluates citizen complaints and performs “hot-spot” sidewalk grinding and

other work that is outside of Jim’s district. While many of his activities remain the most viable

option for quick sidewalk repairs and short-term remedial treatment, they also present

opportunities to communicate more with the public and install some alternative sidewalk

materials.

3.2 Tree-based, Infrastructure-based, and Rootzone-based Solutions

Over time, most cities have come to incorporate trees in parks, along streets, and in subdivisions

and commercial districts. And as city planning and maintenance practices evolve, urban tree

experts have formed strategies to grow large-canopy trees that reap the maximum environmental

benefits despite increasingly constrained environments.

Trees cause less damage when they are provided rootable space, quality soil conditions, and are

situated far from hardscape. Conflicts between tree roots and sidewalks appear to be more related

to growing conditions, tree age, and amount of rooting volume than to species (Randrup et al,

2003); therefore, most of the solutions listed address these problems. It has also been observed

that sidewalk damage can occur without tree roots present, and roots can actually seek out these

cracked areas after they are formed to take advantage of the air spaces (Syndor et al, 2000).

Providing rootable soil under sidewalks can help roots to grow deeper, while alternative surface

materials may reduce or prolong chance of pavement cracking.


Figure 1. Recommended Tree Soil Volumes (Casey Trees).

The Seattle Department of Transportation (SDOT) completed a Trees and Sidewalks Operations

Plan in February of 2015. The plan was developed through a process of thorough research,

community outreach, and inter-departmental collaboration, producing a Solutions Toolkit found

on pages 31-69 of the plan. The toolkit provides the necessary information about almost all of

the recommended materials and strategies in this report. Find the full Solutions Toolkit at

http://www.seattle.gov/transportation/treesandsidewalks_comp.htm or the pdf file directly at

http://www.seattle.gov/transportation/docs/TreeSidewalksOperationsPlan_final215.pdf.

http://www.seattle.gov/transportation/treesandsidewalks_comp.htm

http://www.seattle.gov/transportation/docs/TreeSidewalksOperationsPlan_final215.pdf


Figure 2. Solutions Toolkit overview in the SDOT Trees and Sidewalks Operations Plan.


Figure 3. Solutions Toolkit overview in the SDOT Trees and Sidewalks Operations Plan (continued).


Figure 4. Solutions Toolkit overview in the SDOT Trees and Sidewalks Operations Plan (continued).


Today more than ever before, there are publications and websites providing potential ways to

reduce conflicts between tree roots and infrastructure. Below is the list of tools/strategies with

brief descriptions, cost estimates (feasibility), and useful life expectancy (longevity) from the

SDOT Solutions Toolkit. Each one can be classified as a preventative strategy (prevents

damage), remedial strategy (corrects damage or avoids further damage), or both. Both types

should be incorporated into management plans for a multidimensional approach to management

of sidewalks and the urban forest.

Many of the tools in the toolkit are still being developed and researched, and some are just

starting to be employed in Palo Alto. Alternative surface materials are also scarcely used in Palo

Alto, which forces staff to rely on consumer experiences from other cities with different

environmental influences. Therefore, many of these alternatives in the toolkit need to be

carefully considered before selection on a case by case basis, at least in the first few trials. Below

are the most feasible, promising, and cost-effective tools.

3.2.1 Tree-based solutions

Tree-based solutions, such as species selection and root pruning, focus on action to the tree to

reduce potential for conflicts between trees and infrastructure.

1. Species selection

Species selection is a critical step in determining an appropriate tree for a site with minimal

future sidewalk conflicts. Selecting trees that are most appropriate for a site should consider

constraints such as overhead wires, underground utilities, planter width, required clearances, and

sometimes adjacent property owner preference.

Tree species selection should also account for trunk-flare and root buttress characteristics. Trunk

Diameter at Ground Level (DGL) varies by species, so if the tree trunk has a high DGL and/or

trunk diameter ratio (TDR) it should only be planted in larger planter strips (greater than 3’ wide

and in landscaped areas only). Reference pages 9-13 of Reducing Infrastructure Damage By Tree

Roots: A compendium of Strategies for the DGL Test Survey based on reports from cities in the

San Francisco Bay area.

Of the 230 distinct tree species in the right-of-way in Palo Alto, southern magnolia (Magnolia

grandiflora) and sweetgum (Liquidambar styraciflua) alone make up 23% of the population.

These species pose a significant challenge because they tend to litter the sidewalk with large

debris and surface roots are aggressive and crack and uplift sidewalks. City staff may see these

trees as a nuisance because they have caused recurring issues needing repair, but they also

present an opportunity to test remedial methods. Additionally, their premature removal may


present opportunities to replant with more suitable species and adjacent infrastructure that

prevents future negative impacts.

Figure 5. Camphor (Cinnamomum camphora) (left) and cork oak (Quercus suber) (right) have expanded and

outgrown their narrow and small planting areas in downtown areas. Remedial treatments (sidewalk width reduction and removal) have been employed so far (Palo Alto).

2. Root pruning

Root pruning is a common occurrence when tree roots are uplifting or cracking a part of the

sidewalk that is slated for replacement or repair. Knowledge and understanding of the proper

pruning tools, distance from trunk, timing, frequency, age and condition, trees/conditions to

avoid, and species selection to avoid root pruning are critical to effective decision-making. City

arborists inspecting root pruning activities determine the threshold for root pruning, which is a

challenging task requiring careful consideration of important variables.

Root pruning may also be used to prevent future damage, but should not be used if the arborist

determines that it would significantly impact health or structural integrity of the tree. These


decisions can be difficult to discern, therefore additional staff training and/or field protocols may

add another layer of assurance that best practices are being applied.

Figure 6. City of Palo Alto uses their air spade to employ minimal impacts during tree root excavation prior to

determining allowable root pruning.

3.2.2 Infrastructure-based solutions

Infrastructure-based solutions focus on wise design and materials to reduce conflicts. Two main

strategies include designing planting spaces that are large enough to reduce the proximity of trees

and infrastructure, and using materials that increase the tolerance of concrete cracking/lifting or

replacing concrete with alternative materials.

Design modifications to tree space

1. Increasing planting space

a. Tree pit sizing should allow adequate room for trunk and root growth for the

species of tree being planted. Enlarging tree pits where a tree has outgrown the

area by removing and not replacing hardscape is a viable strategy that Palo Alto

currently uses in some areas.

Figure 7. SDOT estimate of expected useful life (number of years the solution will be effective) and cost of root pruning.


Figure 8. Example of Sidewalk Repair Program marking cracked/lifted concrete adjacent to a street tree to be

removed and not replaced in order to increase the planter length (Palo Alto).

Figure 9. Example where street trees share a continuous planter strip, and landscape trees are set back with a large

landscape area for root growth (Palo Alto).

2. Curving sidewalks

a. Curving sidewalks can be used to meander around planting areas to give trees

more space to grow. They are best used around high-value trees and where there

are numerous planting areas, so it should not be used where space is limited in the

right-of-way.

b. Increasing the radius of the sidewalk around an existing tree is a common practice

in Palo Alto. This practice benefits the tree to an extent, but may have to be

revisited for future root pruning if the tree/roots continue to grow. Fortunately, if

the tree has to be removed, the replacement will automatically have a larger

planting space for trunk growth.


Figure 10. Example of contact stress response with growth over the sidewalk (left) and an example of a different tree

with an increased curve radius (right) to accommodate future trunk growth to hopefully reduce likelihood of future contact (Palo Alto).

Figure 11. City curved the sidewalk around the mature oak’s trunk to provide extra space (Palo Alto).


3. Pop-outs/bulb-outs

a. Radial extensions of a sidewalk at an intersection shorten the crossing distance for

pedestrians and thereby make walking safer. They can also serve as stormwater

treatment and tree growing areas. Using pop-outs/bulb-outs in these public

improvements provide a larger rootable area while benefitting public safety, water

management, and improving the aesthetic quality of streetscapes.

Figure 13. Example of Bioretention planter bulb-outs in Southgate neighborhood (Palo Alto) that could potentially

incorporate trees in the future.

Figure 12. SDOT estimate of expected useful life and cost of curving sidewalks.

Figure 14. SDOT estimate of expected useful life and cost of bulb-outs.


4. Monolithic sidewalks

a. Monolithic sidewalks are a continuous installation; therefore, there is more weight

to resist the uplift of tree roots. It is a more long-term solution, but should not be

used near trees with future vigorous root growth. Numerous neighborhoods

(especially in south Palo Alto) have monolithic sidewalks, which enables new

trees to use soil into larger yards and landscaped areas. However, trees planted

behind monolithic sidewalks are less likely to touch canopies across the street,

which is a marked difference from many neighborhoods with planter strips.

5. Tree islands

a. Tree islands group tree plantings into landscaped areas or in larger parking lot

islands in order to facilitate tree growth in shared soil spaces. This design has the

potential to grow larger trees for shading by grouping trees in a way that mimics a

natural forest with shared soil for roots to grow (eg. Large trees seen in some

medians along El Camino Real in Palo Alto). However, in some situations this

design may compromise some above-ground space that would otherwise be used

for parking and buildings and may not be aesthetically desirable for all architects.

6. Easement (to increase right-of-way space)

a. Easements may allow construction of a sidewalk on private property in order to

provide more space to existing or new street trees. This strategy can provide more

space for the tree, but requires coordination between the City and the property

owner. Palo Alto currently does not have a standardized easement acquisition

policy.

7. Suspended Pavement Systems (AKA Soil Cells)

a. Suspended pavement systems may be used in new tree plantings where there is

not adequate soil volume for root growth. They provide structural support for

placement and rootable soil for trees underneath the hardscape. Soil cell products

can be more expensive than many of the other solutions listed, so they should be

considered more often for CIPs and development projects where there is adequate

funding and constrained sites that preclude open planting areas.

b. The use of suspended pavement systems and structural soil are relatively familiar

for use on commercial properties, but these alternative base materials are not

always cost effective enough for use in public projects. For information about soil

cell use in Palo Alto, reference the Mitchell Park Library and Community Center

parking lot plan.


Figure 15. Deeproot Silva Cells

Figure 16. Cross-sectional view of Silva Cells under hardscape around new street trees during new commercial

development project (2555 Park Blvd, Palo Alto).

Figure 17. Overhead view of Silva Cell installation in public right-of-way under new trees to be planted in curb bulb-outs for new commercial development project (2555 Park Blvd, Palo Alto).


8. Bridging and Ramping

a. Bridging and ramping provide a grade separation between the sidewalk and root

zone of a tree. Numerous techniques exist, including pier and beam bridges,

cantilevered sections, and boardwalks. They are best used to preserve high-value

trees while meeting ADA requirements. Bridging does have height and slope

requirements (e.g. if drop to adjacent grade is greater than 18 inches then the

bridge requires a handrail), and should be non-slip texture. Unfortunately, it can

be expensive and there were no well-known specifications or details to build

bridges over tree roots found during research for this report.

9. Lowered sites

a. Lowered sites provide spatial separation between the finished grade of the

planting pit and the surrounding sidewalk. Tree grates are often installed in

conjunction with lowered sites to provide walkable surface. These are viable

options for areas with high pedestrian volumes with little available planting space,

such as the downtown areas of Palo Alto.

Material modifications – paving and other surface materials

10. Expansion joints

a. Expansion joints may be strategically located near trees to reduce potential of

differential lifting of slabs. Employ if roots can be pruned adjacent to the new

slab, or if there is ample root growing area (eg. wide planter strip or tree is

adjacent to a monolithic sidewalk). Should not be used adjacent to trees with

vigorous root growth and if the goal is a long-term solution.

Figure 18. SDOT estimate of expected useful life and cost of suspended pavement systems.

Figure 19. SDOT estimate of expected useful life and cost of bridging.


11. Thicker slabs

a. A reinforced or thicker sidewalk can be used to help resist the uplift of tree roots.

Reinforcing with steel rebar or wire mesh, or thickening to six to eight inches

thick, may be used adjacent to trees with minimal future root growth and/or

adequate soil volume. However, reinforced pavement may not be allowed in areas

where future utility installation is required or the tree root growth is vigorous.

12. Grinding (beveling)

a. Grinding/beveling is a viable short-term solution with sidewalks lifted less than

one inch. If the lifted portion of the sidewalk can’t be replaced or removed at that

time, beveling is often a cost-effective alternative widely used in Palo Alto.

13. Decomposed granite

a. Decomposed granite (and in some cases mulch) may be used for paths/walkway

surfaces in some residential areas and on top of soil in planting pits. Requires

some maintenance and may be used in areas of low pedestrian traffic where there

is another ADA-compliant route available nearby. Cost is relatively low, and may

be used in areas such as Barron Park neighborhood, and in landscaped

areas/parks.

Figure 20. SDOT estimate of expected useful life and cost of strategically locating expansion joints.

Figure 21. SDOT estimate of expected useful life and cost of installing a thicker concrete slab.

Figure 22. SDOT estimate of expected useful life and cost of beveling.


14. Asphalt shim (wedge)

a. Asphalt shims (or wedges) are used for temporary or interim measures to treat

cracked or lifted sidewalks. This spot treatment may be used when grinding will

not make the sidewalk a safe height, or if removal/replacement are not available

options at the requested time.

Figure 24. Example of asphalt shim used next to street tree (Palo Alto).

15. Porous asphalt

a. Porous asphalt allows water to pass through the pavement. It is best used if

installed along long corridors where concrete sidewalks cannot be constructed and

water infiltration is desired. It should not be used for short segments or if short-

term solutions are desired adjacent to existing trees.

Figure 23. SDOT estimate of expected useful life and cost of decomposed granite.

Figure 25. SDOT estimate of expected useful life and cost of asphalt shims.


16. Pervious concrete

a. Pervious concrete allows air and water to pass through to the soil and bedding

layers below, so it is meant to deter shallow root growth that could uplift regular

concrete. More likely to be used in parking lot areas rather than public sidewalks

adjacent to street trees because it is difficult to repair/replace (poured in place).

See Mitchell Park Library parking lot for example adjacent to

landscaping/stormwater treatment areas.

17. Pavers

a. Pavers come in many materials, colors, and shapes in order to meet different

objectives and site requirements. Rubber sidewalk pavers provide a more flexible

surface than concrete, which in many cases are modular and may be applied

adjacent to trees and pulled up for periodic root pruning and laid back down. Non-

rubber pervious pavers and permeable pavers are often used when stormwater

infiltration is needed, and can be used to cover large or small areas. Maintenance

needs and durability varies by product, but overall pavers should be avoided when

the sidewalk is curved or there are many utility structures that the pavers have to

be cut around.

Figure 26. SDOT estimate of expected useful life and cost of porous asphalt.

Figure 27. SDOT estimate of expected useful life and cost of pervious concrete.


Figure 28. Site preparation prior to rubber sidewalks (Terrecon, Inc) installation adjacent to existing trees for a new

walking path (University Circle, Palo Alto 2016).

Figure 29. Close-up view of rubber sidewalk installation (University Circle, Palo Alto 2016).

Figure 30. SDOT estimate of expected useful life and cost of pavers.


18. Terrewalks®

a. Terrewalks® are modular sidewalks that can be installed as a permanent or

temporary alternative sidewalk surface material. They allow water infiltration and

are made to match typical sidewalk aesthetics. The modular style may enable

future maintenance by simply lifting the pieces, and can be reused at different

sites if employed as temporary sidewalk in areas where needed.

b. These are a financial investment, so City staff should perform cost-benefit

analysis to determine if these are worthwhile compared to other strategies.

c. Additional information can be found on the Terrecon website.

Figure 31. Photo of Terrewalks® installation in Galveston, Texas.

3.2.3 Rootzone-based solutions

Rootzone-based solutions focus on directing (guiding) roots away from infrastructure or creating

conditions that encourage greater root distribution.

1. Root barriers

a. Root barriers are physical barriers (commonly plastic sheeting or interlocking

panels) installed from surface level to a depth of 12 to 24 inches or more. They

are intended to deter root growth near the surface, and are fairly inexpensive.

They should not be installed adjacent to planting areas where the roots are meant

to grow (eg. adjacent to root channels, suspended pavement systems, or large

open planting areas). Typical placement is vertical.

b. Barrier types include deflectors, inhibitors, and traps.

c. Barrier configuration can be linear or circular. Linear is preferred in most cases.


Figure 32. Deeproot linear and surround (circular) root barriers.

Figure 33. Deeproot linear root barriers of various depths.

Figure 34. SDOT estimate of expected useful life and cost of root barriers.


2. Root paths

a. Root paths are narrow trenches, roughly 4 inches wide and 12 inches deep,

installed in compacted subgrade before the gravel base for pavement is added.

Root paths may be installed for new plantings, in areas where tree roots should be

guided around utilities in constrained sites, and as a measure to improve site

conditions for mature high-value trees.

Figure 35. Root path installation radiating out into parking lot to improve the root zone of a high-value tree (Palo Alto).

Figure 36. SDOT estimate of expected useful life and cost of root paths.


3. Structural Soils (as a continuous trench)

a. Structural soils are specially designed to provide nutrients, space, and porosity to

accommodate root growth while enabling compaction to support pavement

without settling. It may be placed to adequate depths to enable root growth

downward and into the soil (usually between 24 inches and 36 inches deep).

b. Engineered Soil Mix (ESM) (Palo Alto’s structural soil) is being required as a

root channel sidewalk base under public sidewalks at many sites in Palo Alto

when new street trees are to be planted as part of development projects.

Specifications are already being drafted by city staff, and this also provides an

opportunity to modify City policy to require this at more sites.

c. Cost comparison:

i. ESM: about $34/cubic foot in 2015 for mix and delivery

ii. Regular soil: $29/cubic foot in 2015 for mix and delivery

Figure 37. Engineered Soil Mix in trench during installation (left) and equipment used to compact the ESM for

engineering standards (right).

Figure 38. SDOT estimate of expected useful life and cost of structural soils.


Figure 39. Draft of a new City of Palo Alto standard detail of a root channel sidewalk base.


4.0 Recommendations

Sustainability and development trends are influencing management strategies across Palo Alto,

thereby necessitating an upgrade of the toolbox for City staff. The daily processes for decision-

making were reviewed and may be improved through recorded and easily accessible decision

trees. With each course of action to be taken, best management practices should be employed by

involved city staff and contractors. Lastly, we recommend strategy packages to be used for

consideration at the early planning stages of Capital Improvement Projects, Development

Review, and the Sidewalk Repair Program.

4.1 City Processes, Planning, and Prioritization

City departments should consider tree health and maintenance one of the top priorities during

any work on sidewalks and other surrounding infrastructure. Taking time to evaluate actions that

preserve existing trees when encountered and incorporate trees in the early planning stages of

projects pays significant dividends over time.

Prioritizing trees can be challenging on constrained sites and streets, but there are opportunities

throughout Palo Alto to incorporate innovative strategies and new materials. The Green

Infrastructure Permit Plan is calling for solutions that will accomplish goals for managing

stormwater and trees. Exploring use of pervious pavers strategically rather than in random small

sites opens options throughout parking lots or piloting use on larger street and sidewalk sections.

Public Works can coordinate activities with Parks and Recreation for maintenance on medians,

and work with project managers for landscaping on other public property.


Figure 40. San Pablo Avenue in Cerrito, CA with surplus of pavement and no trees (top) and the new trees and biotreatment installed in 2011 (bottom).

Communication between Urban Forestry and Transportation may be enhanced by sending an

arborist representative to the Project Kickoff Meetings that happen for all new transportation

CIPs. And there is an open invitation to join the monthly “Coordination Meeting” between

Public Works, Utilities, and Transportation, in which projects are discussed between departments

so everyone knows the upcoming work and avoids issues.

CIPs are currently being evaluated by City staff to determine where there is green infrastructure

potential. Trees have greater potential for long-term growth when they are prioritized at the early

planning stages of a project. Implementing “complete streets” principles is not a thing of the

future, because today smart street design is in the news daily, with trees as an integral piece of

the streetscape.


In addition, many cities including Palo Alto are considering the effects of climate change when

selecting species for new plantings, with emphasis on native and drought-tolerant species

throughout California. When reviewing “thirsty” trees that make up a significant percentage of

the street tree population in Palo Alto, staff should include long-term feasibility when

determining appropriate removal and replacement.

Figure 41. Palo Alto street (downtown) where complete street principles were incorporated to provide a new safe bike lane, planter strip with trees, and stormwater management through cuts in the concrete.

The Urban Forestry group should use GIS and TreeKeeper to track uses of alternative solutions

and recommend sites for future use of the toolkit. Creating forced dropdowns in TreeKeeper for

work history can be a part of the decision process to clearly track all approved activities around

trees such as root pruning, curved sidewalks, adjacent pavement removal, etc. In addition, sites

with potential use of new sidewalk material or shape may be listed and uploaded to GIS for other

departments to see when planning work in the right-of-way.


Another key component to effectively integrating trees into other city plans and priorities is to

include stakeholders. These processes should be transparent and up-to-date for the public. The

city website should be updated to notify people of the public projects and tree removals and

include opportunities to give their input through open response portals. In addition, by posting

links to the toolkit and best management practices online, people can be knowledgeable about

opportunities and be more informed throughout the public outreach process.

4.2 Decision Trees and Best Management Practices

Decision trees and Best Management Practices that are easily accessible enable City staff,

contractors, and the public to better understand and interpret procedures used to determine tree

and sidewalk solutions.

4.2.1 Decision Trees

There is currently a very general procedure that the Public Works Department uses for the

Sidewalk Repair Program, which may be improved by recording the steps and evaluations made

during site assessments. CIPs will also benefit from more detailed guidelines for choosing

solutions that reduce conflicts between trees and sidewalks at the early stages of project planning

and design.

In order to clarify the typical processes and make them more transparent and consistent for staff

and the public, the following diagrams may be used to highlight key decision points. See below

for a summary diagram and the Appendices (7.1) for more detailed decision trees. The decision

trees are intended to work for projects on many scales, including spot treatments, corridor

projects, and more. Program managers should use this process to plan for the necessary time and

resources that are needed to promote tree canopy growth and accessible sidewalks.


Figure 42. SDOT Trees and Sidewalks Operations Plan Decision Process flowchart.

Initial Assessment requires the Program Manager to collect tree information no later than the

30% design or equivalent level. Tree preservation potential, tree mitigation exploration, and

public safety risk should be key components of this analysis. See section 7.1 for the sample

Initial Assessment Form used in Seattle.

Initial Tree Decision requires that an engineer and arborist/landscape architect be together at the

site (ie. project location) to facilitate coordination and sharing information between experts in the

field. They must review the information collected and identify an action to move forward.

Actions include removing the tree and replacing the sidewalk, keeping the tree and maintaining

the sidewalk, or evaluate further. If the tree is unhealthy or hazardous, it should be replaced in

accordance with the Tree Technical Manual Canopy Replacement Standard (Section 3.20) or

minimum 1:1 replacement ratio. Planting should be in the same location, on the same street, or as

close as geographically feasible. If the tree is to be kept and sidewalk maintained, staff should

identify the targeted sidewalk maintenance cycle, estimate cost to achieve lifecycle for repair,

and any tree maintenance needed. The third option, evaluation of the tree and/or sidewalk

further, may be necessary if initial assessments are limited by time or necessary information.

Identifying areas for future actions enables program managers to properly plan the schedule and

budget to assess alternative approaches (ie. alternative sidewalk materials, adjustments to the tree

pit, etc) at problem locations.


Figure 43. Tree Canopy Replacement Standard from the City of Palo Alto Tree Technical Manual.

Further Evaluation should be conducted by a team of professionals with expertise relevant to

the project details. In addition to the technical information, the group should evaluate level of

impact, risks, cost/benefit, anticipated maintenance, public/environmental benefit, community

values, policy guidance, neighborhood context, and historic districts. Palo Alto should

incorporate these criteria and refine them to ensure that best practices are being followed.

Solutions may then be identified for potential use after assessment and evaluation. If a tree is to

be removed, replacement with appropriate placement, soil/water requirements, and species are

key considerations. Using the Toolkit, alternative sidewalk solutions may be used in conjunction

with tree retention and/or replacement, and should provide overall improvement to site

conditions for the future.

Project Implementation requires City Municipal Code and ADA compliance, as well as public

involvement. Public involvement is a key component which is an inherent part of public

improvements with trees and sidewalks. Opportunities for involvement should arise at the initial

tree/sidewalk assessment stage, following the initial assessment, and following selection of

solutions. Public outreach can begin with postings on trees and mailers, and extend through the

project with public forums and design charettes depending on the scope of the project.


Figure 44. SDOT posting notices on trees to communicate plans for tree removal/retention during assessments.

Maintenance of new and repaired sidewalks and trees should be tracked and documented for the

benefit of City staff, contractors, and the public. This tracking will provide information about

durability of materials and life cycle of repair methods, which over time enhances the accuracy

of solution recommendations for a particular site.

4.2.2 Best Management Practices (BMPs)

Being knowledgeable of and equipped to employ the most up to date Best Management Practices

(BMPs) is critical to the success of any coordinated efforts to reduce tree root and sidewalk

conflicts. Appendix B (pages 1-5) of the SDOT Trees & Sidewalks Operations Plan provides a

technical research compilation for BMPs used across the U.S. The City of Palo Alto Tree

Technical Manual also provides guidelines for root pruning, utility work, and other work around

regulated trees.

Certified Arborists Larry Costello and Gary Watson are currently working to write the new Root

Management BMPs, which are expected to be made available to the public through the

International Society of Arboriculture (ISA) in August of 2016. This update will likely provide

the most recent and proven practices and procedures used in the field today.


4.3 Strategy Packages

In a very recent article, James Urban states, “When designing the tree opening, ask yourself what

would be the minimum size of the paving instead of the minimum size of the tree opening”

(Urban, 2016). This tree-centric vision for properties has been employed on many commercial

and public properties throughout Palo Alto, but needs to be promoted for all lands and prioritized

among city staff as a guiding principle when reviewing projects of all kinds. With this mindset,

trees are more likely to get the space they need and other sustainable green components are more

readily applied.

Many of Palo Alto’s sidewalks and trees are managed in a site-by-site manner. This approach has

been successful in some ways, but more efficiency and effectiveness can be achieved with

Strategy Packages made available to staff and the public for review. The following packages are

grouped into the categories: 1) Capital Improvement Plans, 2) Development Review, and

3) Sidewalk Repair Program. Each of these plans/programs differ in budget constraints,

longevity, and city staff involvement, therefore they can be separated and provided general

guidelines for using the tree-based, infrastructure-based, and rootzone-based solutions.

4.3.1 Capital Improvement Plans (CIP)

SDOT conducted three case studies to test their process for evaluating streets in different parts of

the city. With a City Arborist and City Engineer present (at least), the team would determine the

course of action to be 1) keep the tree and repair the sidewalk, 2) remove the tree and repair the

sidewalk, or 3) evaluate further. The plan includes a map of the street (to scale), photos of each

tree/sidewalk site, and a key with descriptions of the treatments and associated costs. There are a

variety of treatments considered at the evaluation and planning stage, providing a multi-faceted

approach to tree and sidewalk management. Treatments include identification of areas for soil

replacement at the tree, sidewalk removal in the planter strip, driveway removal, curb bulb and

ramp upgrades/opportunities, easement negotiations with adjoining property owner, new tree,

shim/beveling, curb realignment opportunity, and trees to be replaced in the future. Additional

information about the case studies can be found on pages 71-73 of the Operations Plan, and

directly at http://www.seattle.gov/transportation/docs/TreeSidewalksAppendices_final215.pdf.

Pilot packages for the city staff to use for early considerations during projects could be used to

compare costs after getting resident’s feedback/preferences at design charrettes and community

meetings. For example, people decide they want a narrower planter, so trees will be smaller and

require and ESM root channel. Another scenario: if people want a large planter they may not

need to incorporate ESM root corridor.

http://www.seattle.gov/transportation/docs/TreeSidewalksAppendices_final215.pdf


Figure 45. SDOT’s case studies included concept plans with detailed assessments and recommendations.

.

Figure 46. Potential uses for rubber sidewalks include replacing concrete around significant landscape trees, such as this valley oak (Quercus lobata) at the Palo Alto History Museum.


4.3.2 Development Review

City staff reviews development projects throughout Palo Alto to ensure code compliance and

enhance customer service. Redevelopment of residential and commercial properties is occurring

rapidly, and City Council has recommended expansion of alternatives to tree removal during

development and expansion of tree protection in municipal codes, policies, and procedures. With

these goals in mind, the Development Services, Planning and Community Environment, and

Public Works Departments play a critical role in code enforcement and development review that

prioritized trees.

The following strategy packages are intended for use by city staff, applicants, and the public. If

made available on the City website, during early planning review, and during building permit

plan check, all involved parties can have access to a variety of solutions that provide for trees

and hardscape. Providing these packages to applicants will enhance transparency and provide

consistent expectations from the City at early stages of project planning and budgeting. Many of

the strategies are currently being employed throughout properties in Palo Alto, but are not

formally written into policy or made widely available.

The two packages provided may be used generally for a) Commercial and b) Residential

development review. A snapshot is provided below, and the appendices provide some standard

details/specifications. Parameters are listed for each type of property, requiring provisions for

trees including minimum rootable soil and best solutions for a given site design. These

parameters allow flexibility during the review process, because the applicant can choose from a

variety of site designs, materials, and tree species that satisfy their preferences, while meeting the

minimum requirements for tree growth.

a) Commercial Development Review

● All commercial zoned properties, including multi-use and Stanford land

● Planning process for Entitlement: Architectural Review Board (ARB), Minor

ARB, Development Review Committee (DRC)

b) Residential Development Review

● New 2-story R-1 and R-2 zoned properties, and Multi-family residential zoned

properties

● Planning process for Entitlement: Architectural Review Board (ARB), Minor

ARB, Individual Review (IR)


Parameter 1 Commercial Development

Review Strategies

Residential Development

Review Strategies

Provide adequate soil

volume per tree: Small

tree (400-600 cubic

feet), medium tree (600-

800 cubic feet), large

tree (800-1200 cubic

feet)

-Incorporate open landscaped area

where trees share soil in enlarged

planters and parking lot islands.

-Install alternative

sidewalk/hardscape base material

(eg. structural soils and soil cells)

adjacent to new street/landscaping

trees where open planters are not

desired*

-Structural soil rooting corridor

under sidewalk adjacent to new

street trees.

-Biaxial geogrid underlayment for

new driveways within the dripline of

street trees and protected trees.


Review Strategies


Review Strategies

Species selection

compatible with site:

buttress, rooting

characteristics, and

size/shape considered

-Choose from Preferred and

Restricted Species List to match the

right tree to the right place.

-Redevelopment should prioritize

larger trees and tree planters when

designing new landscaped areas next

to hardscape.

-Minimize utilities conflicts.

-Choose from Preferred and

Restricted Species List to match the

right tree to the right place. Often

there are more design restrictions, so

for existing trees modify the

sidewalk width and/or material, and

for new trees modify the sidewalk

base and choose a species with

buttress and size that matches the

planter size.


Review Strategies


Review Strategies

Regulated trees to be

protected: protected

trees, street trees, and

designated landscape

trees

-Comply with Tree Technical

Manual and project arborist tree

protection measures


new driveways and other hardscape

within the dripline of street trees and

protected trees.

-Comply with Tree Technical

Manual and project arborist tree

protection measures


new driveways and other hardscape

within the dripline of street trees and

protected trees.



Review Strategies


Review Strategies

Parking lot trees: reach

sufficient size for 50%

shading ordinance

through adequate

rootable soil area and

site design

-Create tree islands for shared

rooting space and stratification of

tree sizes, and prioritize larger

planters where possible.

-Finger-shaped parking lot tree

islands receive structural soil or soil

cells at 24-48-inch depth underneath

the two adjacent parking spaces.

-Parking lot tree planters surrounded

by pavement and/or on top of

parking structures should receive

adequate rooting soil (soil cells

preferably).

-Integrate green infrastructure

(pervious/permeable pavers,

concrete, or asphalt)

-Parking lot tree planters surrounded

by pavement, and/or on top of

parking structures, should receive

adequate rooting soil (soil cells

preferably).

-Integrate green infrastructure

(pervious/permeable pavers,

concrete, or asphalt)


Review Strategies


Review Strategies

Approved landscape

that is acceptable to

other departments:

screening trees/shrubs,

water efficient

landscape, contributes to

on-site stormwater

treatment, compatible

with utilities, ADA

compliant, etc.

-Incorporate appropriate trees and

tree space in the early stages of

project development and review.

Trees should be prioritized and

rooting needs respected during ARB

and DRC review.

-Incorporate appropriate new trees

and tree protection in the early

stages of project development and

review (ie. driveway and utility

placement routed around regulated

trees). Trees should be prioritized

and rooting needs respected during

ARB and IR.

(*)-If the street trees share an open planter strip, deduct soil additions under the hardscape from the final soil goal.

-In areas with more hardscape (ie. downtown and commercial/industrial) alternative base materials will be more

widespread. New large/prominent landscape trees/street trees with roots almost completely covered with hardscape

should be supplied standard planting soil available in soil cells, rather than structural soils.

Table 1. Some potential opportunities to meet parameters for development review.


Figure 47. Architectural drawing of ESM under parking spaces adjacent to tree planters.

Thresholds for changing tree requirements need to be determined for advanced levels of

development. Greater project valuation, size of the property, scope of work, and zoning are some

ways to set up thresholds that trigger higher standards for tree protection and landscaping. Other

reviewing departments currently have policies in place in which a higher sustainable building

practice is triggered once a project’s scope of work exceeds a predetermined amount. The Urban

Forestry staff should work with the Planning and Community Environment Department to assess

the varying requirements for different types of projects. Whether it be that a certain percent of

project budget be solely devoted to tree protection and planting, or require more trees be planted

then removed, staff should discuss innovative potential policies.

4.3.3 Sidewalk Repair Program

The Sidewalk Repair Program has a fairly simple, predominantly reactionary and unsustainable

process for involving City arborists in work where tree roots are encountered. However, the

process may be improved with regular use of an assessment checklist to review trees and

sidewalk repair areas systematically and with a predictable, easily replicated, and transparent

evaluation process. The City of Palo Alto should use the SDOT or similar assessment forms to

craft a checklist for Palo Alto to be used in hard copy form or in TreeKeeper dropdown lists

during site evaluation.

Cost evaluation is another important component to determine which solutions are employed. The

Bicycle Pedestrian (Transportation) budget is not very limited (high funding for infrastructure),

therefore it is a potential source of funding for implementing new strategies. Funding can also

come from grants, but sometimes grant writing is more trouble than it is worth because of the

time and contract staffing required. Ultimately, the PWD should budget appropriately for use of

some more innovative solutions, for the benefit of long-term tree health and the opportunity to

test new strategies that may benefit the City long-term.


Public Works Engineering could initiate a pilot program to test different materials in a receptive

area of the city. For example, in a neighborhood where PW is planning to remove the sidewalk,

plan an interactive section where you install different types of sidewalk surface materials and

encourage viewer feedback to the City website. Another opportunity might be incorporating new

alternative sidewalk surface materials in a recommended package to homeowners via door

hangers or mailers when their street is being assessed for new solutions during the next sidewalk

management plan.

Alternative surface materials such as Terrawalks® may serve as temporary sidewalks in high

traffic areas or where there will be more than a couple days between sidewalk removal and

replacement. The material could be installed on a temporary basis until the work is complete,

then picked up and stored for the next use on another site. If they are well received by the public,

and are more cost-effective than alternatives, they may be installed for long-term use rather than

just temporary projects.

5.0 Conclusion

Many US cities, including Palo Alto, are striving to reach goals for accessibility, health,

environment, equity, efficiency, sustainability, and more. The strategies presented explore ways

to enhance neighborhood streets throughout residential, downtown, and even commercial

districts. In order to facilitate coexistence between substantial trees and hardscape, repair

procedures must be crafted site by site. Utilizing viable Strategy Packages with remedial and

preventative tools, City staff can have access to sophisticated solutions beyond the short list of

options they have long had to reuse.

5.1 Action Items

The process of developing recommendations for this report led to identification of some items

that the City of Palo Alto staff may consider for future action:

● Update the Preferred and Restricted Species List or similar (ie. Canopy Tree Library), to

provide additional information on soil volume, rooting and trunk characteristics, and

minimum allowable tree pit size.

● Integrate tree assessment with other city programs such as asphalt paving, Capital

Improvement Plans, and other right-of-way permits.

● Update standard plans and specifications to align with the tree and sidewalk best

management practices associated with tree pit size, soils, and accessibility requirements.

● Develop forms/online checklists for tree and sidewalk evaluations to track activities for

staff and public review as needed.

● Staff adoption of the decision trees and future consideration of strategy packages for a

variety of different projects now and in the future.


6.0 References

American Forests. (n.d.). Urban Forests Case Studies: Challenges, Potential and Success in a

Dozen Cities. Retrieved from

http://www.americanforests.org/our-programs/urbanforests/urban-forests-case-studies/

Casey Trees. (2008). Tree Space Design: Growing the Tree Out of the Box (pp. 1-15, Rep.).

Washington, D.C. Retrieved from

http://caseytrees.org/wp-content/uploads/2012/02/tree-space-design-report-2008-tsd.pdf

Costello, L. R., & Jones, K. S. (2003). Reducing infrastructure damage by tree roots: A

compendium of strategies. Porterville, CA: Western Chapter of the International Society

of Arboriculture (WCISA).

Davey Resource Group. (2011). City of Palo Alto Right-of-Way Urban Forest Resource Analysis

(Tech.).

Interview with Larry Costello [Telephone interview]. (2015, October 8).

Interview with Jim Amores [Personal interview]. (2015, October 19).

Interview with Dave Dockter [Personal interview]. (2015, October 28).

Interview with Gordon Mann [Telephone interview]. (2015, November 6).

Interview with Matthew Fried [Personal interview]. (2015, November 16).

Interview with Sarah Syed [Personal interview]. (2015, December 8).

Interview with Holly Boyd [Personal interview]. (2015, December 10).

Interview with Bill Croft [Personal interview]. (2016, March 9).

Interview with Kirsten Struve [Personal interview]. (2016, May 18).

Interview with Peg Staeheli [Telephone interview]. (2016, May 16).

Phillips, L. (2016, February). #18 Solutions to the Sidewalk and Tree Roots Conflict. Retrieved

from http://gibneyce.com/18-read-about-sidewalk---tree-roots-conflict.html

http://www.americanforests.org/our-programs/urbanforests/urban-forests-case-studies/

http://caseytrees.org/wp-content/uploads/2012/02/tree-space-design-report-2008-tsd.pdf

http://gibneyce.com/18-read-about-sidewalk---tree-roots-conflict.html

http://gibneyce.com/18-read-about-sidewalk---tree-roots-conflict.html


Sartor, M., Passmore, W., Humble, G., Dockter, D., Krebs, E., Anderson, D., . . . T. (n.d.).

Sustaining the Legacy: City of Palo Alto Urban Forest Master Plan. Retrieved

from http://www.cityofpaloalto.org/civicax/filebank/documents/36187

Seattle Department of Transportation. (2015). Trees and Sidewalks Operations Plan. Retrieved

from


Urban, J. (2013, November 11). An Open Letter to the Complete Streets Movement. Retrieved

from http://www.deeproot.com/blog/blog-entries/an-open-letter-to-the-complete-

streets-movement

Urban, J. (2016, June). Trees, roots & Infrastructure: Designing Tree Openings. International

Society of Arboriculture Arborist News, 25(3), 26-29.

http://www.cityofpaloalto.org/civicax/filebank/documents/36187


http://www.deeproot.com/blog/blog-entries/an-open-letter-to-the-complete-streets-movement

http://www.deeproot.com/blog/blog-entries/an-open-letter-to-the-complete-streets-movement


7.0 Appendices

7.1 Decision Trees and Sample Assessment Forms

Figure 48. Initial Street Tree and Sidewalk Assessment Checklist used by SDOT staff during sidewalk and street tree review (page 1).


Figure 49. Initial Street Tree and Sidewalk Assessment Checklist used by SDOT staff during sidewalk and street tree review (page 2).


Figure 50. SDOT Tree/Sidewalk Assessment and Work Process (expanded).


7.2 Standard Details and Specifications

Many other standard drawings and specifications used by the City of Palo Alto Public Works

Departments can be found at

http://www.cityofpaloalto.org/news/displaynews.asp?NewsID=1834&TargetID=145

Specific vendors/manufacturers also provide this information and sometimes Autocad drawings

for their specific product.

Figure 51. City of Palo Alto tree planting with Engineered Soil Mix standard drawing.

http://www.cityofpaloalto.org/news/displaynews.asp?NewsID=1834&TargetID=145


Figure 52. City of Palo Alto street tree well standard drawing.


Figure 53. City of Palo Alto Standard Drawing for sidewalk construction.


Figure 54. Permeable Interlocking Concrete Pavement Inspection Checklist.


Figure 55. Permeable Interlocking Concrete Pavement Inspection Checklist (continued).






7.3 Material and Vendor Resources

Manufacturers, vendors, installation, and financial costs for some of the recommended solutions.

This is not an all-inclusive list, but an overview of some of the most prominent and local options

for Palo Alto.

Each supplier provides ample information on their website. Phone numbers and email addresses

for sales, services, and technical assistance are available. Other resources include listing of

products and services, research findings, case studies, FAQs, specifications, standard drawings.

1. Local nurseries for tree purchasing

a. Nurseries:

i. Valley Crest (925)862-2485

ii. Boething (650)851-4770

iii. Menlo Growers (408)683-4862

iv. Belmont Nursery (559)255-6860

v. Village Nursery (916)364-2945

vi. Mid Valley Trees (559)734-4641

vii. Western Tree (408)842-4892

viii. Calaveras Nursery (209)772-1823

ix. Bonfante Nursery (408)840-7143

b. Tree installation should be completed by qualified City of Palo Alto staff (Urban

Forestry group) or ISA Certified Arborists/teams overseen by a Certified Arborist.

c. Cost varies by size and species. Larger trees cost more than smaller trees, and

species availability may affect price (ie. trees in high demand but with limited

supply may be more expensive than trees that are widely available at most

nurseries).

2. Root Pruning

a. Root pruning activities should only be performed by a Certified Arborist.

b. Expected cost should cover personnel time, and the City already owns most of the

necessary equipment (hand saws, root shaving device, pneumatic or hydro device

to remove soil and expose roots without digging).

3. Structural Soils - TMT Enterprises, Inc - www.tmtenterprises.net/products.php

4. Suspended pavement systems (ie. soil cells)

a. Deeproot - www.deeproot.com

i. Silva Cell

ii. Silva Cell 2

b. Citygreen – www.citygreen.com

i. Strata Cell

ii. Stratavault

c. Greenblue – www.greenblue.com

http://www.tmtenterprises.net/products.php

http://www.deeproot.com/

http://www.citygreen.com/

http://www.greenblue.com/


i. StrataVault

ii. GrassRings

iii. RootCell

iv. StrataCell

5. Pervious pavers, permeable pavers and pervious concrete

6. Recycled rubber sidewalk

a. Terrecon Inc – www.terrecon.com

i. Rubbersidewalks™

ii. Verlayo®

7. Terrewalks®

a. Terrecon Inc – www.terrecon.com

8. Biaxial geogrid underlayment – various suppliers

a. Example pricing: 13.12’ x 246’ Tensar type (143 pounds): $548.49 +tax

9. Root barriers – various suppliers

http://www.terrecon.com/

http://www.terrecon.com/

Documents

Reducing Tree Root and Sidewalk Conflicts: Analysis and